The present invention relates to phosphor blends for use in discharge lamps. In particular, the present invention relates to phosphor blends useful for achieving high color rendering index (“CRI”) in mercury discharge lamps. The present invention also relates to high-CRI fluorescent lamps.
A phosphor is a luminescent material that absorbs radiation energy in a portion of the electromagnetic spectrum and emits energy in another portion of the electromagnetic spectrum. Phosphors of one important class are crystalline inorganic compounds of very high chemical purity and of controlled composition to which small quantities of other elements (called “activators”) have been added to convert them into efficient fluorescent materials. With the right combination of activators and host inorganic compounds, the color of the emission can be controlled. Most useful and well-known phosphors emit radiation in the visible portion of the electromagnetic spectrum in response to excitation by electromagnetic radiation outside the visible range. Well-known phosphors have been used in mercury vapor discharge lamps to convert the ultraviolet (“UV”) radiation emitted by the excited mercury vapor to visible light. Other phosphors are capable of emitting visible light upon being excited by electrons (used in cathode ray tubes) or X rays (for example, scintillators in X-ray detection systems).
The efficiency of a lighting device that uses a phosphor increases as the difference between the wavelengths of the exciting radiation and that of the emitted radiation narrows. In low-pressure mercury discharge lamps (also commonly known as fluorescent lamps), excited mercury atoms in the discharge, upon returning to the ground state, mainly emit UV radiation having wavelength of 254 nm (about 12% of the emitted radiation having wavelength of 185 nm). Ideal phosphor for mercury discharge lamps should absorb the 254 nm and 185 nm strongly and convert the absorbed radiation efficiently. Effort, therefore, has been expended to produce phosphors for these lamps to be excited by radiation having wavelengths as close to 254 nm as possible. Three or four phosphors are typically included in a low-pressure mercury discharge lamp to provide white light that simulates sunlight. Different blends of phosphors can produce fluorescent lamps with different color temperatures. The color temperature of a light source refers to the temperatures of a blackbody source having the closest color match to the light source in question. The color match is typically represented and compared on a conventional CIE (Commission International as I'Eclairage) chromaticity diagram. See, for example, “Encyclopedia of Physical Science and Technology,” Vol. 7, 230-231 (Robert A. Meyers (Ed.), 1987). Generally, as the color temperature increases, the light becomes bluer. As the color temperature decreases, the light appears redder. Typical incandescent lamps have color temperature of about 2700 K while fluorescent lamps have color temperature in the range of 3000-6500 K. When the point representing the light source is not exactly on the black body locus of the CIE chromaticity diagram, the light source has a correlated color temperature, which is the temperature on the black body locus which would give nearly the same color to the average human eye.
In addition to color temperature, color rendering index (“CRI”) is another important characteristic of the light source. CRI is a measure of the degree of distortion in the apparent colors of a set of standard pigments when measured with the light source in question as opposed to a standard light source. CRI depends on the spectral energy distribution of the emitted light and can be determined by calculating the color shift; e.g., quantified as tristimulus values, produced by the light source in question as opposed to the standard light source. Under illumination with a lamp with low CRI, an object does not appear natural to the human eye. Thus, the better light sources have CRI close to 100. Typically, for color temperatures below 5000 K, the standard light source used is a blackbody of the appropriate temperature. For color temperatures greater than 5000 K, sunlight is typically used as the standard light source. Light sources having a relatively continuous output spectrum, such as incandescent lamps; typically have a high CRI; e.g., equal to or near 100. Light sources having a multi-line output spectrum, such as high pressure discharge lamps, typically have a CRI ranging from about 50 to 80. Fluorescent lamps typically have a CRI in the range of 75-85. Typically, fluorescent lamps have higher color temperature, but lower CRI than incandescent lamps. In general lighting applications, it is desirable to provide light sources having color temperature in the range of 4000-6000 K; i.e., in the range of color temperature of fluorescent lamps. Thus, it is very desirable to provide fluorescent lamps that have higher CRIs and still maintain higher color temperature than that of typical incandescent lamps. In addition, there is a continued need to provide phosphor compositions that are excitable in the region near 254 nm and emit in the visible range such that they may be used flexibly to design light sources having tunable properties, such as color temperatures and CRI.